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</table></div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../language.html" title="cpp/language">C++ 语言</a></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="expressions.html" title="cpp/language/expressions"> 表达式</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> 概述 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="value_category.html" title="cpp/language/value category"> 值类别</a>（左值 lvalue、右值 rvalue、亡值 xvalue）</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="eval_order.html" title="cpp/language/eval order"> 求值顺序</a>（序列点）</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constant_expression.html" title="cpp/language/constant expression"> 常量表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#.E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/expressions"> 不求值表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="expressions.html#.E5.88.9D.E7.AD.89.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/expressions"> 初等表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lambda.html" title="cpp/language/lambda">lambda 表达式</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 字面量 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="integer_literal.html" title="cpp/language/integer literal">整数字面量</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="character_literal.html" title="cpp/language/character literal">字符字面量</a>，包含<a href="escape.html" title="cpp/language/escape">转义序列</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="nullptr.html" title="cpp/language/nullptr">空指针字面量</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="user_literal.html" title="cpp/language/user literal">用户定义字面量</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 运算符 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_assignment.html" title="cpp/language/operator assignment"> 赋值运算符</a>:
<code>a=b</code>, <code>a+=b</code>, <code>a-=b</code>, <code>a*=b</code>, <code>a/=b</code>, <code>a%=b</code>, <code>a&amp;=b</code>, <code>a|=b</code>, <code>a^=b</code>, <code>a&lt;&lt;=b</code>, <code>a&gt;&gt;=b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_incdec.html" title="cpp/language/operator incdec"> 自增与自减</a>: <code>++a</code>, <code>--a</code>, <code>a++</code>, <code>a--</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_arithmetic.html" title="cpp/language/operator arithmetic"> 算术运算符</a>:
<code>+a</code>, <code>-a</code>, <code>a+b</code>, <code>a-b</code>, <code>a*b</code>, <code>a/b</code>, <code>a%b</code>, <code>~a</code>, <code>a&amp;b</code>, <code>a|b</code>, <code>a^b</code>, <code>a&lt;&lt;b</code>, <code>a&gt;&gt;b</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_logical.html" title="cpp/language/operator logical"> 逻辑运算符</a>: <code>a||b</code>, <code>a&amp;&amp;b</code>, <code>!a</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_comparison.html" title="cpp/language/operator comparison"> 比较运算符</a>: <code>a==b</code>, <code>a!=b</code>, <code>a&lt;b</code>, <code>a&gt;b</code>, <code>a&lt;=b</code>, <code>a&gt;=b</code>, <code>a&lt;=&gt;b</code><span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_member_access.html" title="cpp/language/operator member access"> 成员访问运算符</a>: <code>a[b]</code>, <code>*a</code>, <code>&amp;a</code>, <code>a-&gt;b</code>, <code>a.b</code>, <code>a-&gt;*b</code>, <code>a.*b</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_other.html" title="cpp/language/operator other"> 其他运算符</a>: <code>a(...)</code>, <code>a,b</code>, <code>a?b:c</code> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_comparisons.html" title="cpp/language/default comparisons">默认比较</a><span class="t-mark-rev t-since-cxx20">(C++20)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_alternative.html" title="cpp/language/operator alternative">运算符的代用表示</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operator_precedence.html" title="cpp/language/operator precedence">优先级和结合性</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="fold.html" title="cpp/language/fold">折叠表达式</a><span class="t-mark-rev t-since-cxx17">(C++17)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new">new 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete">delete 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="throw.html" title="cpp/language/throw">throw 表达式</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="alignof.html" title="cpp/language/alignof">alignof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof.html" title="cpp/language/sizeof">sizeof</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="sizeof....html" title="cpp/language/sizeof...">sizeof...</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">typeid</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="noexcept.html" title="cpp/language/noexcept">noexcept</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="operators.html" title="cpp/language/operators">运算符重载</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> 类型转换 </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="implicit_cast.html" title="cpp/language/implicit conversion">隐式转换</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast">const_cast</a></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="explicit_cast.html" title="cpp/language/explicit cast">显式转换</a> <code>(T)a</code>, <code>T(a)</code></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="cast_operator.html" title="cpp/language/cast operator">用户定义转换</a></td></tr>
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<p>查询类型的信息。
</p><p>用于必须知晓<a href="object.html#.E5.A4.9A.E6.80.81.E5.AF.B9.E8.B1.A1" title="cpp/language/object">多态对象</a>的<a href="incomplete_type.html#.E5.8A.A8.E6.80.81.E7.B1.BB.E5.9E.8B" title="cpp/language/type">动态类型</a>的场合以及静态类型鉴别。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<td> <code><b>typeid(</b></code> <span class="t-spar">类型</span> <code><b>)</b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
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<td> <code><b>typeid(</b></code> <span class="t-spar">表达式</span> <code><b>)</b></code>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
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<p>必须在使用 <code>typeid</code> 前包含头文件 <a href="../header/typeinfo.html" title="cpp/header/typeinfo">&lt;typeinfo&gt;</a>（若不包含该头文件，则每次使用关键词 <code>typeid</code> 都令程序非良构。）
</p><p>typeid 表达式为<a href="value_category.html" title="cpp/language/value category">左值表达式</a>，指代一个具有<a href="static.html" title="cpp/language/static">静态存储期</a>的，多态类型 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 或某个其派生类型的对象。
</p>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<div class="t-li1"><span class="t-li">1)</span> 指代一个表示 <span class="t-spar">类型</span> 类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 对象。若 <span class="t-spar">类型</span> 为引用类型，则结果所指代的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 对象表示被引用的类型。</div>
<div class="t-li1"><span class="t-li">2)</span> 检验表达式 <span class="t-spar">表达式</span></div>
<div class="t-li2"><span class="t-li">a)</span> 若 <span class="t-spar">表达式</span> 为标识某个<a href="object.html#.E5.A4.9A.E6.80.81.E5.AF.B9.E8.B1.A1" title="cpp/language/object">多态类型</a>（即声明或继承至少一个<a href="virtual.html" title="cpp/language/virtual">虚函数</a>的类）对象的<a href="value_category.html" title="cpp/language/value category">泛左值表达式</a>，则 <code>typeid</code> 表达式对该表达式求值，然后指代表示该表达式动态类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 对象。若该泛左值表达式为通过对一个指针应用一元 * 运算符所得，且该指针为空指针值，则抛出 <span class="t-lc"><a href="../types/bad_typeid.html" title="cpp/types/bad typeid">std::bad_typeid</a></span> 类型或从 <span class="t-lc"><a href="../types/bad_typeid.html" title="cpp/types/bad typeid">std::bad_typeid</a></span> 派生的类型的异常。</div>
<div class="t-li2"><span class="t-li">b)</span> 若 <span class="t-spar">表达式</span> 不是多态类型的泛左值表达式，则 <code>typeid</code> <a href="expressions.html#.E4.B8.8D.E6.B1.82.E5.80.BC.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/expressions">不对该表达式求值</a>，而它所指代的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 对象表示该表达式的静态类型。不进行左值到右值、数组到指针或函数到指针转换。<span class="t-rev-inl t-since-cxx17"><span>然而对于纯右值参数，（形式上）要进行<a href="implicit_cast.html#.E4.B8.B4.E6.97.B6.E9.87.8F.E5.AE.9E.E8.B4.A8.E5.8C.96" title="cpp/language/implicit conversion">临时量实质化</a>：typeid 确定其结果对象的类型。</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span></div>
<p>在所有情况下，typeid 都忽略 cv 限定符（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typeid</span><span class="br0">(</span>T<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">typeid</span><span class="br0">(</span><span class="kw4">const</span> T<span class="br0">)</span></span></span>）
</p><p>若 <code>typeid</code> 的操作数为类类型或到类类型的引用，则该类类型必须不是<a href="incomplete_type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B" title="cpp/language/type">不完整类型</a>。
</p><p>若对处于构造和销毁过程中的对象（在构造函数或析构函数之内，包括构造函数的<a href="constructor.html" title="cpp/language/initializer list">初始化器列表</a>或<a href="data_members.html#.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96" title="cpp/language/data members">默认成员初始化器</a>）使用 <code>typeid</code>，则此 <code>typeid</code> 所指代的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a></span></span> 对象表示正在构造或销毁的类，即便它不是最终派生类。
</p>
<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
<p><a href="../keyword/typeid.html" title="cpp/keyword/typeid"><tt>typeid</tt></a>
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>当应用于多态类型的表达式时，typeid 表达式的求值可能涉及运行时开销（虚表查找），其他情况下 typeid 表达式都在编译时解决。
</p><p><code>typeid</code> 所指代的对象的析构函数是否在程序结束时执行是未指明的。
</p><p>不保证同一类型上的 typeid 表达式的所有求值都指代同一个 <span class="t-lc"><a href="../types/type_info.html" title="cpp/types/type info">std::type_info</a></span> 实例，不过这些 type_info 对象的 <span class="t-lc"><a href="../types/type_info/hash_code.html" title="cpp/types/type info/hash code">std::type_info::hash_code</a></span> 相同，其 <span class="t-lc"><a href="../types/type_index.html" title="cpp/types/type index">std::type_index</a></span> 也相同。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">const</span> <a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a><span class="sy3">&amp;</span> ti1 <span class="sy1">=</span> <span class="kw1">typeid</span><span class="br0">(</span>A<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">const</span> <a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a><span class="sy3">&amp;</span> ti2 <span class="sy1">=</span> <span class="kw1">typeid</span><span class="br0">(</span>A<span class="br0">)</span><span class="sy4">;</span>
 
<a href="../error/assert.html"><span class="kw774">assert</span></a><span class="br0">(</span><span class="sy3">&amp;</span>ti1 <span class="sy1">==</span> <span class="sy3">&amp;</span>ti2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 不保证</span>
<a href="../error/assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>ti1.<span class="me1">hash_code</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">==</span> ti2.<span class="me1">hash_code</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 保证</span>
<a href="../error/assert.html"><span class="kw774">assert</span></a><span class="br0">(</span><a href="../types/type_index.html"><span class="kw447">std::<span class="me2">type_index</span></span></a><span class="br0">(</span>ti1<span class="br0">)</span> <span class="sy1">==</span> <a href="../types/type_index.html"><span class="kw447">std::<span class="me2">type_index</span></span></a><span class="br0">(</span>ti2<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 保证</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><p>该示例展示使用一种实现时的输出，其中 type_info::name 返回完整类型名；若使用 gcc 或相似者则须通过 c++filt -t 过滤。</p><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;typeinfo&gt;</span>
 
<span class="kw1">struct</span> Base <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 非多态</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Base2 <span class="br0">{</span> <span class="kw1">virtual</span> <span class="kw4">void</span> foo<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 多态</span>
<span class="kw1">struct</span> Derived2 <span class="sy4">:</span> Base2 <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <span class="kw4">int</span> myint <span class="sy1">=</span> <span class="nu0">50</span><span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> mystr <span class="sy1">=</span> <span class="st0">"string"</span><span class="sy4">;</span>
    <span class="kw4">double</span> <span class="sy2">*</span>mydoubleptr <span class="sy1">=</span> nullptr<span class="sy4">;</span>
 
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"myint has type: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>myint<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"mystr has type: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>mystr<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span>
              <span class="sy1">&lt;&lt;</span> <span class="st0">"mydoubleptr has type: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>mydoubleptr<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="co1">// std::cout &lt;&lt; myint 为多态类型的泛左值表达式；求值</span>
    <span class="kw4">const</span> <a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a><span class="sy3">&amp;</span> r1 <span class="sy1">=</span> <span class="kw1">typeid</span><span class="br0">(</span><a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> myint<span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span> <span class="sy1">&lt;&lt;</span> <span class="st0">"std::cout&lt;&lt;myint has type : "</span> <span class="sy1">&lt;&lt;</span> r1.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="co1">// std::printf() 不是多态类型的泛左值表达式；不求值</span>
    <span class="kw4">const</span> <a href="http://zh.cppreference.com/w/cpp/types/type_info"><span class="kw446">std::<span class="me2">type_info</span></span></a><span class="sy3">&amp;</span> r2 <span class="sy1">=</span> <span class="kw1">typeid</span><span class="br0">(</span><a href="../io/c/fprintf.html"><span class="kw1845">std::<span class="me2">printf</span></span></a><span class="br0">(</span><span class="st0">"%d<span class="es1">\n</span>"</span>, myint<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"printf(<span class="es1">\"</span>%d<span class="es1">\\</span>n<span class="es1">\"</span>,myint) has type : "</span> <span class="sy1">&lt;&lt;</span> r2.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="co1">// 非多态左值时为静态类型</span>
    Derived d1<span class="sy4">;</span>
    Base<span class="sy3">&amp;</span> b1 <span class="sy1">=</span> d1<span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"reference to non-polymorphic base: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>b1<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    Derived2 d2<span class="sy4">;</span>
    Base2<span class="sy3">&amp;</span> b2 <span class="sy1">=</span> d2<span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"reference to polymorphic base: "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span>b2<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
 
    <span class="kw1">try</span> <span class="br0">{</span>
        <span class="co1">// 解引用空指针：对于非多态表达式 OK</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"mydoubleptr points to "</span> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span><span class="sy2">*</span>mydoubleptr<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> 
        <span class="co1">// 解引用空指针：对多态左值则不行</span>
        Derived2<span class="sy2">*</span> bad_ptr <span class="sy1">=</span> nullptr<span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"bad_ptr points to... "</span><span class="sy4">;</span>
        <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="kw1">typeid</span><span class="br0">(</span><span class="sy2">*</span>bad_ptr<span class="br0">)</span>.<span class="me1">name</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span> <span class="kw1">catch</span> <span class="br0">(</span><span class="kw4">const</span> <a href="../types/bad_typeid.html"><span class="kw448">std::<span class="me2">bad_typeid</span></span></a><span class="sy3">&amp;</span> e<span class="br0">)</span> <span class="br0">{</span>
         <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">" caught "</span> <span class="sy1">&lt;&lt;</span> e.<span class="me1">what</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">myint has type: int
mystr has type: std::basic_string&lt;char, std::char_traits&lt;char&gt;, std::allocator&lt;char&gt; &gt;
mydoubleptr has type: double*
50
std::cout&lt;&lt;myint has type : std::basic_ostream&lt;char, std::char_traits&lt;char&gt; &gt;
printf("%d\n",myint) has type : int
reference to non-polymorphic base: Base
reference to polymorphic base: Derived2
mydoubleptr points to double
bad_ptr points to...  caught std::bad_typeid</pre></div></div> 
</div>

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